Abstract

Quick, what is your favorite color of M&Ms® candy? Do you want to know what dyes were used to make that color? Check out this science project to find out how you can do some scientific detective work to find out for yourself.

Objective

Use paper chromatography to see which dyes are used in the coatings of your favorite colored candies.

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Introduction

Have you ever had a drop of water spoil your nice print-out from an inkjet printer? Once the water hits the paper, the ink starts to run. The water is absorbed into the fibers of the paper by capillary action. As the water travels through the paper, it picks up ink particles and carries them along. This same process that spoils a perfect print-out can also be put to good use. There is even a name for it: paper chromatography.

Chromatography is a group of techniques, including paper chromatography, that are used to separate the various components in a complex mixture or solution. In each chromatography apparatus there is generally a mobile phase, which is a fluid the solution is dissolved in, and a stationary phase, which is a material the fluid moves through. For example, in paper chromatography, water is the mobile phase and filter paper is the stationary phase. The mobile phase is also called the solvent.

How does the chromatography setup separate the components in the solution? The components ideally move at different speeds as they travel through the stationary phase. This is done by adjusting the mobile and stationary phases so that they interact with different properties of the solution's components, such as their molecular size, electrical charge, or other chemical properties, to distinguish and separate them from each other. In paper chromatography, different pigments can be separated out from a solution based on the solubility of the pigments. A pigment that is more soluble (or more hydrophilic) than another pigment will generally travel farther because it will be easier for it to dissolve in the mobile phase (water) and be carried with the mobile phase along the stationary phase (filter paper). A pigment that is less soluble (or more hydrophobic), or interacts more with the filter paper than the water, will generally travel a shorter distance.

Another example of a chromatography system is a glass column filled with tiny, inert beads (the stationary phase). The solution to be separated is added to the column, and is then "washed out" with some type of fluid (the mobile phase). In this case, the separation is based on molecular size. Smaller molecules will pass through the spaces between the beads more easily, so they will come out of the column more quickly. Larger molecules will take more time to pass between the beads, so they will come out of the column later. You can separate the smaller molecules from the larger molecules by collecting the liquid that comes off such a column in a series of separate containers.

You can probably now imagine how chromatography can be used to separate (purify) specific components from a complex mixture and identify chemicals, for example crime scene samples like blood, drugs, or explosive residue. Highly accurate chromatographic methods are used for process monitoring, for example to ensure that a pharmaceutical manufacturing process is producing the desired drug compound in pure form.

In paper chromatography, you can see the components separate out on the filter paper and identify the components based on how far they travel.
To do this, we calculate the retention factor (Rf value) of each component. The
Rf value is the ratio between how far a component travels and the distance the solvent travels from a common starting point (the origin). For example, if one of the sample components moves 2.5 centimeters (cm) up the paper and the solvent moves 5.0 cm, as shown in Figure 1 below, then the
Rf value is 0.5. You can use Rf values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. In Figure 1, the light blue shading represents the solvent and the dark blue spot is the colored solution sample.

Figure 1. values are how different components are compared to each other in paper chromatography.

Rf values are calculated by looking at the distance each component travels on the filter paper compared to the distance traveled by the solvent front. This ratio will be different for each component due to its unique properties, primarily based on its adhesive and cohesive factors.

When measuring the distance the component traveled, you should measure from the origin (where the middle of the spot originally was) and then to the center of the spot in its new location. To calculate the
Rf value, we then use Equation 1 below.

Equation 1:

[Please enable JavaScript to view equation]

In our example, this would be:

[Please enable JavaScript to view equation]

Note that an Rf value has no units because the units of distance cancel.

In this food science project, you will use the Rf value to compare the "unknown" components of colored candy dyes with the "known" components of food coloring dyes. Since there are only a small number of approved food dyes, you should be able to identify the ones used in the candies by comparison to the chromatography results for food coloring.

Terms and Concepts

Capillary action

Paper chromatography

Solution

Mobile phase

Stationary phase

Solvent

Solubility

Hydrophilic

Hydrophobic

Retention factor (Rf)

Questions

Why do different compounds travel different distances on the piece of paper?

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Experimental Procedure

Do your background research so that you are knowledgeable about the terms, concepts, and questions listed in the Background tab.

Choose three colors of candies you want to test.

For example, you could test red M&Ms®, brown M&Ms®, and blue Skittles®.

Cut each filter paper in half (length-wise) to make approximately 2 centimeters (cm) wide by 7.5 cm long strips. You will need at least 30 chromatography strips.

Use a pencil to lightly label which candy color or food coloring will be spotted on each paper strip. Label 5 chromatography strips for each candy color and 5 strips for each food coloring (red, green, and blue). Tip: do not use a pen for writing on the strips: the ink will run when the solvent passes through the strips.

Draw a pencil line 1 cm from the edge of each strip of paper, as shown in Figure 2 below.

This will be the origin line.

You will spot the candy color for each strip right on this line, as shown in Figure 2.

Figure 2. Each chromatography strip will have an origin line. The dye to be tested will be spotted in the middle of the origin line.

Next you need to extract some dye from each candy you wish to test.

Fill the 100 mL beaker with some water.

Use the pipet to put a single drop of water in the clean plate or plastic lid as shown in Figure 3 below. Set one candy in the drop of water.

Tip: If you use too much water, the dye will not be concentrated enough to see on the chromatography strip.

How to use the pipet: Squeeze the pipet at its widest point. While continuing to squeeze, insert the narrow end into the beaker of water. Release the wide end and the pipet will fill with water. Put the narrow end of the pipet directly over the petri dish. Gently squeeze the wide end of the pipet to release one drop of water.

Leave the candy in the drop of water for three minutes to allow the dye to dissolve.

Remove the candy, then dip a pipet tip, or clean wooden splint tip, into the now-colored drop of water.

Spot the candy dye solution onto the chromatography strip by touching the pipet tip, or a wooden splint, to the strip, right in the center of the origin line as shown in Figure 4 below.

Allow the spot on the strip to dry completely (this should take approximately 1 minute).

Repeat steps 6e to 6f three more times. You want to make sure to have enough dye on the chromatography strip so that you can see the dye components when they separate out on the paper.

Repeat steps 6b to 6g with four more strips and four new candies that are the same type and color (e.g., all red M&Ms®).

Figure 3. To extract the candy dye, leave a piece of candy in a single drop of water for three minutes. When you remove the candy, a puddle of dye will be left behind.

Figure 4. Spot the extracted candy dye onto the paper chromatography strips using the tip of a wooden splint or a pipet.

Repeat step 6 for the other two colors of candy you want to test. In the end you should have 15 spotted chromatography strips— 5 for each colored candy type.

You also need to prepare chromatography strips with food coloring dyes.

These will be your known compounds, with which you will compare the "unknown" candy dyes.

For each food coloring color put a drop of coloring in the bottom of the petri dish.

Dip a clean wooden splint tip or a pipet into the drop of food coloring.

Spot the food coloring onto a chromatography strip by touching the wooden splint, or pipet, to the strip, right in the center of the origin line.

Add 1/8 teaspoon of salt to 4 cups of water (approximately 1 gram [g] of salt to 1 liter [L] of water).

If you only have a ¼ teaspoon measuring spoon fill that spoon half full of salt— that will be close enough for this project.

Shake or stir until the salt is completely dissolved.

Pour a small amount of the salt solution into the 500 mL beaker.

Clip two of the prepared chromatography strips to a wooden splint. Make sure the two strips do not touch each other or the beaker and that their bottoms are aligned. Rest the splint on top of the beaker so that the strips hang straight into the beaker.

If necessary, add more of the salt solution. The goal is to have the end of the chromatography strips just touching the surface of the solvent solution (salt solution), as shown in Figure 5 below.

Figure 5. Your chromatography setup should look similar to this example. The edge of the chromatography strips should just barely touch the solvent.

Figure 5. Your chromatography setup should look similar to this example. The edge of the chromatography strips should just barely touch the solvent.

Let the solvent rise up the strip (by capillary action) until it is about 0.5 cm from the top then remove the strip from the solvent. Keep a close eye on your chromatography strip and the solvent front— if you let it run too long the dye may run off the paper and become distorted.

Tip: Use Equation 1, which is given in the Introduction (located in the Background tab), for calculating the Rf value.

Repeat steps 10–13 until you have run all of the chromatography strips.

Each time you run the experiment make sure there is enough solvent in the beaker. The chromatography strips should be just touching the surface of the solvent. Add more solvent (salt solution) as needed.

Using the five repeated strips for each candy color (or food coloring), calculate the average Rf for each dye component.

Analyzing Your Results

Create a data table like Table 1 for each candy type and color or food coloring that you tested in your lab notebook.

Candy Type/Color or Food Color:

Component Color

Component Rf value

Total number of components:

Table 1. Data table in which to record each of the separated components from one type and color of candy or food color.

Record all your results for one candy type and color or food color in a different data table.

Make a pie chart for each candy type and color as well as food color. The pie chart should show the number of components (one wedge per color), the color of each component (label and color each wedge appropriately), and the Rf value for each component (part of the wedge's label).

Compare the Rf values for the candy colors and the food coloring dyes. Can you identify which food coloring dyes match which candy colors? How many dye components does each candy color have? Do your results make sense to you?

Hint: You can look at the ingredients on the packaging to see which food coloring dyes may have been used to help you answer these questions.

Note: It is possible that other components in the candies may affect how well the food coloring dyes travel through the paper. Why do you think this might be? (Hint: Think about solubility and re-read this part of the Introduction.) If you have unexpected results, do you think this might help explain them?

Troubleshooting

If you like this project, you might enjoy exploring these related careers:

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Variations

Try this project with a variety of candies— for example, does the red in Skittles® look the same as the red in M&Ms® when processed with chromatography? Is the average Rf value nearly the same? Look in the ingredients on each package to try and determine if the same dyes were used.

You could try this science project again but this time compare using different kinds of solvents (e.g., salt water, water, vegetable oil, isopropyl rubbing alcohol, etc.). Does a dye travel different distances depending on the solvent you use? What do you think this tells you about the solubility of that dye in the different solvents?

Do the dyes you tested in this science project travel differently on different kinds of filter paper? You could repeat this project to try and find out. For example, you could compare lightweight paper towels, heavyweight paper towels, white coffee filter papers, and other types of filter paper. Do they all work? Do some work better than others? Why do you think this is?

For more advanced chromatography experiments, see these Science Buddies projects:

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Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.

Q: What kind of paper will work for doing this science project?

A: This project works best with high quality filter paper, like that found in the Candy Chromatography Science Kit or specialty chromatography paper (which is more expensive). Coffee filters and regular printer paper will not work. It is possible to use paper towels to just see the colors separate, but the degree of separation is low and the results will be hard ro quantify. For more details, see the Science Buddies webpage on
Paper Chromatography Resources.

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The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

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